Directional couplers are used for measuring high frequency signals and the like.
FIG. 7(A) is a typical block diagram of a radio frequency (RF) transmitter circuit 100 of a cellular phone device or the like. The RF transmitter circuit 100 includes an antenna 111, a directional coupler 120A, a transmission power amplifier 113, a modulation circuit 112, and a detection circuit 114. The directional coupler 120A is of a transmission line type, and includes a main line 121 and a coupling line (secondary line) 122. The main line 121 is connected between the antenna 111 and the transmission power amplifier 113. The detection circuit 114 is connected to the secondary line 122 of the directional coupler 120A and controls the transmission power amplifier 113 based on a signal from the secondary line 122 that couples to the main line 121.
FIG. 7(B) and FIG. 7(C) are equivalent circuit diagrams of the directional coupler 120A. Here, it is assumed that the directional coupler 120A is an ideal circuit in which a magnetic-field coupling coefficient (Km) of a mutual inductance M formed between the main line 121 and the secondary line 122 is equal to one. The main line 121 is connected to a signal input port RFin and a signal output port RFout at its two end portions. The signal input port RFin is connected to the transmission power amplifier 113. The signal output port RFout is connected to the antenna 111. The secondary line 122 is connected to a coupling port CPL and an isolation port ISO at its two end portions. The coupling port CPL is connected to the detection circuit 114. The isolation port ISO is connected to a termination resistor. The main line 121 and the secondary line 122 are electrically coupled to each other through a distributed capacitance (coupling capacitance) C between two lines and magnetically coupled to each other through a mutual inductance M.
As illustrated in FIG. 7(B), when a signal S1 is inputted from the signal input port RFin, electric-field coupling causes a signal S2 to propagate in a direction to the coupling port CPL and a signal S3 to propagate in a direction to the isolation port ISO in the secondary line 122. Further, magnetic-field coupling causes a signal S4 and a signal S5 to propagate in a direction from the isolation port ISO to the coupling port CPL in a closed loop formed of the secondary line 122 and ground (GND). The signals S2 and S4 flowing to the coupling port CPL are aligned in phase. Powers of the signals S2 and S4 are added together, and a resulting signal is outputted from the coupling port CPL. On the other hand, the signals S3 and S5 flowing at the isolation port ISO are in opposite phase. Powers of the signal S3 and the signal S5 cancel out each other at the isolation port ISO. Thus, the output power of the RF transmitter circuit 100 may be detected from the output of the coupling port CPL of the directional coupler 120A.
Further, as illustrated in FIG. 7(C), when a signal S6 is inputted to the signal output port RFout due to reflection from the antenna or the like, the electric-field coupling causes a signal S7 to propagate in a direction to the coupling port CPL and a signal S8 to propagate in a direction to the isolation port ISO in the secondary line 122. Further, the magnetic-field coupling causes a signal S9 and a signal S10 to propagate in a direction from the coupling port CPL to the isolation port ISO. The signals S8 and S10 flowing to the isolation port ISO are aligned in phase. Powers of the signals S8 and S10 are added together, and a resulting signal is outputted from the isolation port ISO. On the other hand, the signals S7 and S9 flowing at the coupling port CPL are in opposite phase. Thus, powers of the signal S7 and the signal S9 cancel out each other at the coupling port CPL. Accordingly, an effect of the signal S6 due to the reflection from the antenna or the like does not reach to the coupling port CPL but reaches only to the isolation port ISO. Typically, the isolation port ISO is connected to a termination resistor. However, recently in some cases, the isolation port ISO has been connected to a detection circuit for detecting the reflection from the antenna and controlling the RF transmitter circuit. Note that, when detecting the reflection from the antenna, names and functions of respective ports (signal input port, signal output port, coupling port, and isolation port) change. However, in the following description, each port name is kept the same as for the transmission signal.
As described above, in the directional coupler for use in a RF communication circuit, the coupling capacitance C corresponds to the electric-field coupling coefficient (Kc), and the mutual inductance M corresponds to the magnetic-field coupling coefficient (Km). In the ideal directional coupler, the electric-field coupling coefficient (Kc) and the magnetic-field coupling coefficient (Km) are both equal to one. Thus, it is possible to cancel out the signal due to the electric-field coupling and the signal due to the magnetic-field coupling completely at the isolation port or the coupling port. However, in an actual directional coupler, there is parasitic inductance due to a peripheral circuit such as routing wiring, wires, or the like, and it is difficult to set the value of the magnetic-field coupling coefficient (Km) to one as described above. Thus, it is hard to cancel out the signal due to the electric-field coupling and the signal due to the magnetic-field coupling completely, and it is difficult to achieve ideal directivity of the directional coupler.
The signal due to the electric-field coupling and the signal due to the magnetic-field coupling may be cancelled out completely at the isolation port or the coupling port by adjusting (decreasing) the electric-field coupling coefficient (Kc) in response to a decrease in the magnetic-field coupling coefficient (Km) of the directional coupler. However, it is necessary to make some changes in physical structure such as widening a line gap between the main line and the secondary line or the like in order to decrease the electric-field coupling coefficient (Kc). Such changes in physical structure may increase the size of the directional coupler or cause a further change or decrease in the magnetic-field coupling coefficient (Km). Thus, the adjustment of the electric-field coupling coefficient (Kc) should be avoided as much as possible.
Therefore, in some cases, a load circuit with an adjustable impedance (for example, see Japanese Unexamined Patent Application Publication No. 01-274502 is used in place of the termination resistor that is to be connected to the isolation port. FIG. 8(A) is a circuit diagram illustrating an exemplary configuration of a directional coupler to which a load circuit is connected. This directional coupler 120B is provided with a load circuit 123 at the isolation port ISO. The load circuit 123 includes a resistor R, an inductance L, and a capacitor C, which are connected in parallel between the isolation port ISO and a ground potential. In this load circuit 123, the impedance may be varied by adjusting the resistor R or the inductance L or the capacitor C. This enables improvement in the directivity of the directional coupler 120B.
Note that, in some cases, an attenuator is added at the secondary line of the directional coupler (for example, see Japanese Unexamined Patent Application Publication No. 2009-044303). FIG. 8(B) is a circuit diagram illustrating an exemplary configuration of a directional coupler including attenuators. In this directional coupler 120C, attenuators 124A and 124B are connected to the coupling port CPL and the isolation port ISO to remove effects of mismatching at the coupling port CPL and the isolation port ISO.